Photographic elements relying on silver halide emulsions for image recording have been recognized to possess outstanding sensitivity to light for more than a century. Roentgen discovered X radiation by the inadvertent exposure of a silver halide photographic element. In 1913 the Eastman Kodak Company introduced its first product specifically intended to be exposed by X radiation.
The desirability of limiting patient exposure to high levels of X radiation has been recognized from the inception of medical radiography. In 1918 the Eastman Kodak Company introduced the first medical radiographic product which was dual coated that is, coated with silver halide emulsion layers on the front and back of the support.
At the same time it was recognized that silver halide emulsions are more responsive to light than to X rays. The Patterson Screen Company in 1918 introduced matched intensifying screens for Kodak's first dual coated (Duplitized.RTM.) radiographic element. An intensifying screen contains a phosphor which absorbs X radiation and emits radiation in the visible spectrum or in an adjacent spectral region--i.e., the ultraviolet or infrared.
Research Disclosure, Vol. 184, August 1979, Item 18431, summarizes the state of the art of constructing radiographic elements, including dual coated radiographic elements, prior to the commercial introduction of thin tabular grain (T-Grain.RTM.) emulsions in radiographic elements. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
By 1979 rapid access processing of radiographic elements was well established. In rapid access processing an imagewise exposed radiographic element is introduced into a processor and emerges in 1 to 2 minutes fully processed and dry to the touch. Rapid access processing is illustrated by Barnes et al U.S. Pat. No. 3,545,971.
Successful rapid access processing requires limiting the drying load that is, the water ingested by the hydrophilic colloid layers, principally the silver halide emulsion layers, that must be evaporated to produce a dry image bearing element. One possible approach is to foreharden the radiographic element fully, thereby reducing swelling (water ingestion) during processing. Because silver image covering power (maximum density divided by the silver coating coverage) of pre-T-grain radiographic elements was markedly reduced by forehardening of the radiographic elements, it was the accepted practice not to foreharden the radiographic elements fully, but to complete hardening of radiographic elements during rapid access processing by incorporating a pre-hardener, typically glutaraldehyde, in the developer.
Since silver halide emulsions require hydrophilic colloid for their preparation and full forehardening of emulsion layers leads to marked reductions in silver covering power, reduction of the drying load on the rapid processors has been achieved by limiting the hydrophilic colloid content of radiographic elements. However, when the hydrophilic colloid content of the emulsion layer falls too low, the problem of wet pressure sensitivity is encountered. Wet pressure sensitivity is the appearance of graininess produced by applying pressure to the wet emulsion during development. In rapid access processing the radiographic element passes over guide rolls, which are capable of applying sufficient pressure to the wet emulsion during development to render any wet pressure sensitivity propensity of a radiographic element manifest, particularly if any of the guide rolls are in less than optimum adjustment.
An image sharpness limitation of dual coated radiographic elements results from light emitted by each intensifying screen passing through the transparent film support to expose the silver halide emulsion layer on the opposite side of the support to light. A variety of conventional pre-T-Grain techniques for reduction of crossover are set out in Research Disclosure, Item 18431, cited above, Section V. These crossover reduction techniques have the disadvantage of
(a) reducing radiographic imaging speed,
(b) increasing the minimum density of the image,
(c) interfering with successful rapid access processing, and/or
(d) complicating the manufacture or use of the radiographic elements.
As a result, in 1979 radiographic elements used commercially routinely exhibited crossover levels in excess of about 20 percent.
One approach suggested prior to 1979 for crossover reduction was to dissolve a filter dye in one or more of the hydrophilic colloid layers forming the radiographic element. Such dyes must, of course, be selected to minimize residual density (stain) in the image bearing radiographic element. A pervasive problem with dissolved dyes has been their migration to the latent image forming silver halide grains, whether coated directly in the image forming emulsion layers or in underlying layers. This has resulted in loss of photographic speed, which, of course, runs directly counter to the general aim in adopting a double coated radiographic element format in the first instance. Thus, where this approach has been followed, a balance of dye desensitization and residual crossover has been accepted. Although mordants have been employed to reduce dye migration, they have not been effective in preventing loss of photographic speed or have increased dye stain. The dissolved dye approach to crossover reduction is illustrated by Doorselaer U.K. Pat. Spec. No. 1,414,456 and Bollen et al U.K. Pat. Spec. No. 1,477,638 and 1,477,639.
To reduce dye migration to the image forming silver halide grains a variant approach has been to adsorb the dye to the surfaces of silver halide grains other than those employed in imaging. This approach reduces speed loss, but has the disadvantage of requiring silver halide grains to be present in addition to those required for latent image formation, thereby significantly increasing silver coverages. Further, an added silver halide grain population increases hydrophilic colloid requirements and correspondingly increases drying times. Millikan et al U.K. Pat. Spec. No. 1,426,277 illustrates this approach applied to a specialized photographic imaging system in which a silver halide grain population is present in addition to the grain population which is relied upon to produce a latent image.
It has not in general been found feasible to employ in radiographic elements to reduce crossover filter layers of the type found in other photographic elements. For example, the most common filter layers encountered in color photography, Carey Lea silver layers, are unacceptable in radiographic elements, since they leave high residual stain levels. Lemahieu et al U.S. Pat. No. 4,092,168 discloses the use of particulate filter dyes in photographic elements, but contains no mention of utility in radiographic elements.
When T-Grain emulsions (emulsions in which tabular grains having a thickness of less than 0.30 .mu.m account for greater than 50 percent of the total grain projected area and have an average aspect ratio of greater than 5:1) were introduced, the performance characteristics of radiographic elements, particularly dual coated radiographic elements, improved markedly in a number of respects. Kofron et al U.S. Pat. No. 4,439,520 taught that chemically and spectrally sensitized T-Grain emulsions improve image sharpness (in ways unrelated to crossover) and improve the speed-granularity relationship of the emulsions. T-Grain emulsions are known to exhibit superior covering power levels when compared to other emulsions of comparable speed. As a corollary, less silver is required to match the performance of other emulsions. Dickerson U.S. Pat. No. 4,414,304 taught that T-Grain emulsions are more resistant to loss of covering power on forehardening. This allows radiographic elements to be fully forehardened or at least forehardened to a greater extent, reducing the risk of film damage in handling and allowing processing to be simplified by the reduction or elimination of prehardening. Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,425 taught that substantially optimally spectrally sensitized T-Grain emulsions are capable of reducing crossover into the 15 to 20 percent range without resorting to any of the conventional crossover reduction techniques taught by Item 18431, Section V, or incurring their attendant disadvantages.